1. Field of the Invention
The present invention relates to charge sensitive preamplifiers which are used to convert the charge signals from semiconductor radiation detectors to voltage pulses, that are in turn shaped and analyzed for pulse height, the pulse height being proportional to the energy deposited in the detector by each photon or particle striking the detector and, more particularly, but not by way of limitation to a novel method of re-setting the preamplifier which is fast, free of spurious effects, and which adds little noise to the signal. The new method can be used with detectors requiring either positive or negative bias voltage.
2. Background Art
Semiconductor radiation detectors produce a current pulse with an integrated charge that is proportional to the energy deposited in the detector by each photon or particle interaction. Charge sensitive amplifiers (preamplifiers) are used to convert this charge to a voltage pulse for further shaping, amplification, and analysis. The quality of the preamplifier signal plays a large role in the performance of the detector. Specifically, preamplifier noise is a strong contributor to resolution degradation. The resolving ability, i.e., the ability of a radiation detector to distinguish or resolve small differences in the energy levels of photons or particles is the paramount measure of detector quality. For this reason, preamplifier design and manufacture is rich in both proprietary and public prior art In its simplest embodiment, a charge sensitive preamplifier is a closed-loop amplifier with a capacitive feedback element. Charge deposited at the input node unbalances the amplifier that responds by making a step function change in output sufficient to inject an equal but opposite charge at the input nodexe2x80x94thereby re-balancing the circuit.
In the usual event that signals continue to occur one after another, the preamplifier will eventually saturate, that is, the output voltage will reach the limit of the dynamic range of the amplifier and no further signals can be processed. To remedy this situation, a large value resistor is added in parallel with the feedback capacitor. This resistor provides a continuous discharge path for the charge stored in the capacitor and, thus, the preamplifier will remain in operation as long as the detector current does not exceed the current capacity (given the output voltage limit of the preamplifier) of the feedback resistor.
Unfortunately, the feedback resistor is a source of noise, which degrades the resolution of the detector. There have been many innovations in preamplifier design to overcome the feedback resistor noise problem including optical feedback, pulsed optical feedback, and transistor reset methods. U.S. Pat. No. 5,347,231 and T. Lakatos, G. Hegyesi, and G. Kalinda, Nucl. Instr. and Meth., A378, pg. 683 (1996), contain detailed overviews of preamplifier reset techniques and refer to a number of relevant publications.
A number of other references describe various methods of resetting. These include:
Optical feedback in U.S. Pat. No. 3,611,173. This technique results in an output signal whose shape varies with count rate and which cannot be shaped and processed with integrity.
Pulsed-optical feedback in D. A. Landis, F. S. Goulding, and J. M. Jakelvic, Nucl. Inst. and Meth., 87, pg. 211 (1970); and D. A. Landis et al., IEEE Trans. Nucl. Sci., NS-18 (1), pg. 115 (1971). Disadvantages are that the light must be isolated from the detector element, the JFET employed can take a long time for full recovery following illumination, and the circuit does not work for positively biased detectors.
Transistor reset preamplifier in D. A. Landis et al., IEEE Trans. Nucl. Sci., NS-29 (1), pg. 619 (1982). The additional capacitance of the JFET employed on the input of the preamplifier exacts a heavy penalty in noise, so these preamplifiers are not suitable for low energy detectors where noise plays a large role in detector resolution.
Switches that are integrated in the JFET itself as well as JFETs having an additional electrode serving as an injector to provide reset current in U.S. Pat. No. 5,170,229. Such innovations cannot be used with the great variety of commercially available JFETs that are useful for the range of detectors in common use.
Resetting by forward biasing the gate-source junction in V. Radeka, IEEE Trans. Nucl Sci., NS-17 (3), pg. 433 (1970). Proper operation of this circuit depends on the detector capacitance which may not be stable and with some detectors the electrode structure may prevent efficient reset and may cause polarization of secondary electrodes as a result of resetting.
Resetting by forward biasing the drain-gate junction of the JFET in N. W. Madden et al., IEEE Trans. Nucl, Sci., NS-37 (2), pg. 171 (1990). Because of the circuit arrangement, it is not possible to incorporate a circuit to limit the drain voltage excursion during reset.
In addition, European Patent Application No. 89300335.0, titled JUNCTION FIELD EFFECT TRANSISTORS, describes a device that may be used as a high impedance charge or current amplifier and which may be used to restore charge.
Accordingly, it is a principal object of the invention to provide a preamplifier that overcomes previous methods and employs a new method of re-setting that is fast.
It is a further object of the invention to provide such a preamplifier that is free of spurious effects.
It is an additional object of the invention to provide such a preamplifier that adds little noise to the signal.
It is another object of the invention to provide such a preamplifier that can be used with detectors requiring either positive or negative bias voltages.
Other objects of the invention will become apparent from, or will be elucidated in, the following description and on the accompanying drawing figures.
The present invention achieves the above objects, among others, by providing, in a preferred embodiment, a charge sensitive preamplifier for a radiation detector, comprising: an amplifier having a JFET input (stage) and a capacitive feedback element, said amplifier producing an output voltage (pulse) proportional to a charge (pulse) deposited at said JFET input by said radiation detector; and circuitry connected to said amplifier output and to a source node of said JFET to provide to said source node a pulsed reset signal.